Method of control slope regulation and control slope regulation apparatus

ABSTRACT

A control loop has a control slope associated therewith. The control loop is provided to control a unit under control. A method of regulating the control slope comprises the step of measuring the control slope of the control loop and modifying a parameter associated with the unit under control in order to maintain the control slope within a desired range. Lock of the control loop is therefore maintained.

FIELD OF THE INVENTION

This invention relates to a method of regulating a control slope of, forexample, a control loop associated with a unit under control, such as apower amplifier. This invention also relates to a control sloperegulation apparatus for regulating a control slope of, for example, acontrol loop associated with a unit under control, such as a poweramplifier.

BACKGROUND OF THE INVENTION

In the field of Radio Frequency (RF) communications, wirelesscommunications devices in a wireless communications network, for examplea cellular telecommunications network, possess transceiver circuits. Itis known that transceiver circuits comprise, inter alia, a transmittercircuit.

Typically, a wireless communications network is designed and built so asto comply with one or more communications standards. One example of acommunications standard is the Global System for Mobile communications(GSM) standard, which imposes strict requirements upon the function of aMobile Subscriber (MS) handset that constitutes a wirelesscommunications device. Of course, other Time Division Multiple Access(TDMA) communications standards exist as well as standards for othermultiple access schemes.

In relation to the GSM standard (and others), an MS handset can transmiton a given channel. However, limitations are imposed upon power that canbe generated in adjacent channels when transmitting on the givenchannel. In this respect, a so-called Switching Output Radio FrequencySpectrum (SxORFS) specification is associated with operation of thetransmitter circuit, which operates in a burst mode.

Hence, it is important to maintain power control for the transmittercircuit throughout a transmission in order to comply with, inter alia,the SxORFS specification. In some operating conditions associated withthe transmitter circuit, deviation from the SxORFS specification canoccur, for example when a power amplifier of the transmitter circuit“ramps down” from a saturated state.

One known control loop circuit comprises a reference ramp generatorarranged to generate a digital signal that is used to control a poweramplifier that amplifies an input data signal to be transmitted by thetransceiver circuit via an antenna. The digital signal has a profilethat ramps up, maintains a level for a predetermined period of time andthen ramps down again. The digital signal is fed to adigital-to-analogue converter and then low-pass filtered to yield areference voltage signal. The reference voltage signal is fed to asummation unit that also receives a negative detection voltage signal.An output of the summation unit, constituting an error signal, iscoupled to a controller that implements a proportional-integral controlalgorithm in order to yield an automatic power control voltage signalfor controlling a bias of the power amplifier. A sample of an outputsignal of the power amplifier is obtained using a directional coupler,the sample of the output signal being processed by a detection unitcapable of generating the detection voltage signal that is a measure ofthe power generated by the power amplifier, expressed as a voltagesignal.

When operating, the power amplifier can run hot, a battery providing asupply voltage can be running low or the power amplifier can betransmitting in certain frequency “corners” that result in the poweramplifier being unable to achieve a maximum output power of, forexample, 33 dBm, when required to do so. This results in a persistenterror signal due to a persistent difference between the referencevoltage signal and the detection voltage signal. The controllertherefore continually attempts to change the automatic power controlvoltage signal in order to try to achieve a maximum output power at theoutput of the power amplifier, and so the output power of the amplifiersaturates and the bandwidth of the control loop collapses to zero; thisstate is known herein as “hard” saturation. Consequently, when theoutput power of the power amplifier ramps down at the end of a burst,the power amplifier must first “wind down” from the saturated state, andsuch winding down requires time to do so. However, in order to complywith the communications standard, the output power of the transmittercircuit has to ramp down within a predetermined period of time and aprofile of the ramp down has to possess a predetermined shape, forexample a raised-cosine function profile.

Of course, if a proportion of the time allotted for ramp-down is used towind the power amplifier down, the power amplifier has to complete theramp-down according to the raised-cosine profile in the remaining (less)time and so the ramp-down has a steeper gradient than would otherwise bethe case if it was not necessary to unwind from the saturated state.Consequently, the likelihood of out-of-band interference beinggenerated, i.e. in adjacent channels, is increased.

One known partial solution to the generation of the out-of-bandinterference is disclosed in US patent publication no. US 2004/0176049A1, where first and second error signals are generated and used whenapplying a limit reference input (AOC_MAX) of a controller of a radiocommunications transmitter. However, the circuit proposed in US2004/0176049 A1 does not cure generation of the out-of-band interferencein some circumstances.

In some instances, the circuit can achieve a target output power so thata detected error between actual output power and the target output poweris zero. When this occurs, the power amplifier can be sufficientlycompressed such that satisfactory SxORFS performance for a ramp downevent cannot be guaranteed. In this respect, upon reaching the targetoutput power, the bandwidth of a control loop comprising the poweramplifier collapses to a non-zero value below a minimum threshold valueand so the loop is unable to maintain lock; this state is known hereinas “soft” saturation. Alternatively, another saturation state can occurwhen the output power reaches the target output power, but the bandwidthof the control loop collapses to zero; this state is known herein as“virtual” saturation. Hence, it can be seen that bandwidth and thus lockof the control loop are not always guaranteed in relation to the circuitof US 2004/0176049.

STATEMENT OF INVENTION

According to the present invention, there is provided a method ofregulating a control slope and a control slope regulation apparatus asset forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one embodiment of the invention will now be described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a control slope regulation apparatusconstituting an embodiment of the invention;

FIG. 2 is a graph of control slope performance associated with theapparatus of FIG. 1;

FIG. 3 is a graph of a control voltage signal deviation versus time; and

FIG. 4 is a graph of output power versus time.

DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout the following description identical reference numerals willbe used to identify like parts.

Referring to FIG. 1, a reference ramp generator 100 is coupled to aDigital-to-Analogue Converter (DAC) 102, the DAC 102 being coupled to acontrol loop 104, in particular a first low-pass filter 106 of thecontrol loop 104. In the control loop 104, the low-pass filter 106 iscoupled to a first input of a summation unit 108, an output of thesummation unit 108 being coupled to an input of a primary control unit110. In this example, the primary control unit 110 implements aproportional-integral control algorithm.

A bias control circuit 112 of a power amplifier 114 is coupled to anoutput of the primary control unit 110 and constitutes a unit undercontrol. In this example, the power amplifier 114 is part of atransmitter sub-circuit of a transceiver circuit of an RF communicationsdevice, such as a cellular telecommunications handset. Of course, theunit under control need not be the power amplifier 114 and can be anysuitable device, circuit or otherwise (referred to as a “plant” incontrol theory parlance) that needs to be controlled.

In this example, a directional coupler 116 is coupled to an output ofthe power amplifier 114 and is also coupled to a detection unit 118, forexample a log detector. The detection unit 118 is also coupled to asecond input of the summation unit 108.

In order to provide control slope regulation functionality, the outputof the primary control unit 110 is also coupled to a feedback pathcomprising an Analogue-to-Digital Converter (ADC) 120, the ADC 120 beingcoupled to a Sinc filter 122. The Sinc filter 122 of the feedback pathis coupled to a control slope regulator apparatus 124, in particular aninput of a second low-pass filter 126.

An output of the second low-pass filter 126 is coupled to an input of ahigh-pass filter 128 in the feedback path as well as a first input 129of a secondary control unit 130. An output of the high-pass filter 128is coupled to a second input 131 of the secondary control unit 130 inthe feedback path. A regulation output of the secondary control unit 130is coupled to a regulation input of the reference ramp generator 100.

In operation, in order for the RF communications device to transmitinformation, a transmission burst has to be generated by the transmittersub-circuit of the transceiver. The reference ramp generator 100therefore generates a normalised reference signal, the normalisedreference signal being scaled by the reference ramp generator 100 toyield a reference voltage signal 132. In this respect, an ‘external’scaling value is provided to a ‘PWR’ scaling input (not shown) of thereference ramp generator 100 in order to provide the degree of scalingof the normalised reference signal calibrated as required to achieve atarget output power at the output of the power amplifier 114.

In this example, the reference voltage signal 132 has a profile fordriving the power amplifier 114 in accordance with Gaussian MinimumShift Keying (GMSK) bias control for the GSM standard. Consequently, theprofile comprises a ramp-up portion 134, an active portion 136 and aramp-down portion 138, the ramp-up and ramp-down portions having aso-called “raised cosine” shape. Typically, the active portion 136 islevel. The skilled person will, of course, appreciate that otherprofiles can be employed for other applications. However, in order to beable to measure changes to a control slope of the control loop 104, thereference ramp generator 100 is arranged to amplitude modulate theactive portion 136, i.e. after ramp-up and before ramp-down portions134, 138. The provision of the amplitude modulation constitutesproviding a perturbation on the active portion 136.

The generated reference voltage signal is a digital signal that isconverted to the analogue domain by the DAC 102, the analogue referencevoltage signal then being low-pass filtered by the first low-pass filter106. The filtered analogue reference signal (hereinafter referred to asthe reference voltage, V_(ref)) is combined with a negative feedbacksignal V_(det), in the summation unit 108.

The summation unit 108 outputs an error signal that is used by theprimary control unit 110 in order to modify an automatic power controlvoltage signal, V_(apc), constituting a control voltage signal. In thisexample, the primary control unit 110 has a frequency response, G_(c).The control signal is used to set the bias control circuit 112 in orderto achieve the target output power at the output of the power amplifier114.

By virtue of the directional coupler 116, a portion of the output signalof the power amplifier 114 is tapped off and represented by thedetection unit 118 as the feedback voltage signal, V_(det), having alike slope and scale to that of the reference voltage, V_(ref). Thefeedback voltage signal, V_(det), has a negative amplitude in order toachieve a subtraction at the summation unit 108.

The control voltage signal, V_(apc), is also converted back to thedigital domain by the ADC 120, the digitised control voltage signal,V_(apc)(n), then being filtered by an optional digital filter, forexample the Sinc filter 122. Thereafter, the control slope regulatorapparatus 124 processes the filtered digitised control signal,V_(apc)(n) in order to measure the control slope of the control loop104.

In this respect, the control signal, V_(apc)(n), is effectivelyband-pass filtered by the second low-pass filter 126 and the high-passfilter 128, but the second low-pass filter 126 and the high-pass filter128 have been employed separately in order to extract a low-passfiltered version of the digitised control signal, V_(apc)(n), for thefirst input 129 of the secondary control unit 130. A band-pass filteredversion of the digitised control signal, ∂V_(apc)(n), is provided at thesecond input 131 of the secondary control unit 130. The secondarycontrol unit 130 then processes the band-pass filtered version of thedigitised control signal, ∂V_(apc)(n), in order to measure the controlslope of the control loop 104 and determine whether the control slope iswithin a desired range. As described later herein, the low-pass filteredversion of the digitised control signal, ∂V_(apc)(n), is used to handlea special case where the power amplifier 114 “hard” saturates duringramp up and before the perturbation can be applied.

If the control slope is not within the desired range, secondary controlunit 130 generates a regulation signal, sat_bo, in order to instruct thereference ramp generator 100 to “back-off” the power generated by thepower amplifier 114. This is achieved by the reference ramp generator100 modifying the scaling applied to the normalised reference signalmentioned above.

It has been discovered that the control slope of the control loop 104relates directly to the ability of the control loop 104 to maintain lockon a target output power. In this respect, it has been noted that thebandwidth of the control loop 104, a factor that dictates the ability ofthe control loop 104 to maintain lock, “collapses” in a proportionalmanner with collapse of the control slope. In order to be able tomeasure the control slope of the control loop 104, the secondary controlunit 130 processes the low-pass version and the band-pass version of thedigitised control signal, ∂V_(apc)(n) in a manner that will be describedhereinbelow.

Information regarding the control slope is available from the frequencyresponse between the control voltage signal, V_(apc), and the referencesignal, V_(ref):

$\frac{V_{apc}}{V_{ref}} = {\frac{G_{c}}{1 + G_{ol}} = \frac{G_{c}}{1 + {k_{p}k_{d}G_{c}G_{p}G_{d}}}}$

Where: G_(c) is the frequency response of the primary control unit 110,

-   -   G_(ol) is the open-loop frequency response of the control loop        104,    -   G_(p) is the normalised small signal linearised frequency        response of the power amplifier 114,    -   G_(d) is the normalised small signal linearised frequency        response of the detection unit 118,    -   k_(p) is the small signal DC response of the power amplifier 114        at the output power of the power amplifier 114,    -   k_(d) is the small signal DC response of the detection unit 118        at the output power of the power amplifier 114.

At low frequency, the magnitude of the frequency response of the primarycontrol unit 110 tends towards infinity (|G_(c)|→∞) due to the presenceof the integral part of the closed loop control algorithm implemented bythe primary control unit 110. Additionally:

$ {\lim\limits_{\omegaarrow 0}\frac{V_{apc}}{V_{ref}}}arrow\frac{1}{k_{p}k_{d}G_{p}G_{d}}  = \frac{1}{k_{p}k_{d}}$

Hence, at low frequencies, the frequency response between the controlvoltage signal, V_(apc), and the reference voltage signal, V_(ref), isinversely proportional to the control slope, the product k_(p)k_(d). Theresponse of the control voltage signal, V_(apc), to the perturbationplaced on the reference voltage signal, V_(ref), can therefore be usedto extract information regarding the control slope, when theperturbation is at low frequency.

Referring to FIG. 2, and based upon the limit equation, for a givenlevel of perturbation on the reference signal, V_(ref), the controlvoltage signal, V_(apc), deviates in a manner inversely proportional tothe control slope. However, since the use of the perturbations canadversely affect key transmission performance characteristics associatedwith the power amplifier 114, for example phase error and modulationORFS (mod ORFS), the frequency, amplitude and duty cycle ofperturbations are programmable. In this example, the perturbationstranslate to a 0.1 dB change to the output power of the power amplifier114.

Typically, 10 bit resolution is employed by the reference ramp generator100 in order to generate the reference voltage signal, V_(ref). However,in this example, in order to control the amplitude of the perturbationswith greater accuracy, 11 bit resolution is employed. The perturbationshave, in this example, a frequency in a range between about 10 kHz andabout 50 kHz. The duty cycle of the perturbations can be between about50% and about 1%. Further, the perturbations are only applied, asmentioned above, during the active portion of the burst profile.Typically, the active portion 136 is provided with the perturbationsonly when the power amplifier 114 is required to achieve output powersclose to a maximum possible output power for the power amplifier 114,for example greater than 31 dBm in respect of a maximum output power of33 dBm. Consequently, when the perturbations are not being provided,elements constituting the feedback path mentioned above can be powereddown in order to reduce power consumption of the circuit.

Since the band-pass filtered version of the control voltage signal,V_(apc)(n) corresponds to a computation of the deviation of the controlvoltage signal, ∂V_(apc), the secondary control unit 130 processes theband-pass filtered version of the control voltage signal, ∂V_(apc), inorder to determine whether the control slope of the control loop 104 isabove a pre-set threshold level, V_(T). In the event that the deviationof the control voltage signal, ∂V_(apc), exceeds the threshold level,V_(T), the control slope is deemed to be collapsing along with thebandwidth of the control loop 104; this collapse occurs when the poweramplifier 114 saturates and so the control voltage signal, V_(apc),applied to the bias control circuit 112 needs to be backed off byapplication of the regulation signal, sat_bo, to the reference rampgenerator 100 by the secondary control unit 130.

For a special case where the power amplifier 114 saturates on ramp-upand the control voltage signal, V_(apc), has reached a maximum limitthereof, the low-pass filtered version of the control voltage signal,V_(apc), is processed by the secondary control unit 130, as thedeviation of the control voltage signal, ∂V_(apc), simply suggests aninfinite control slope. Since this is clearly not the case, thesecondary control unit 130 monitors the absolute value of the controlvoltage signal, V_(apc), and if it exceeds an upper bound it can beconcluded that the power amplifier 114 has “hard” saturated duringramp-up and so corrective action can be taken to back-off the outputpower of the power amplifier 114. When this hard saturation occurs,application of the perturbations is temporarily disabled.

Referring to FIGS. 3 and 4, deviations of the control voltage signal,∂V_(apc), can occur when so-called “slope droop” takes place across theburst implemented by the power amplifier in an attempt to follow theprofile of the normalised reference signal. Slope droop can occur whenthe charge on a battery of an electronic device, such as the cellulartelephone handset mentioned above, is nearing depletion.

Consequently, whilst the deviations of the control voltage signal,∂V_(apc), in respect of a first perturbation 150 and a secondperturbation 152, in the present example, do not exceed the thresholdlevel, V_(T), a third perturbation 154 does exceed the threshold level.This behaviour translates into the following output power, P_(out),performance of the power amplifier 114. In respect of the first andsecond perturbations 150, 152, the output power, P_(out), of the poweramplifier 114 remains at a maximum level of 33.0 dBm. However, inrespect of the third perturbation 154, the secondary control unit 130calculates that the third perturbation 154 has exceeded the thresholdlevel, V_(T), and so generates the regulation signal, sat_bo. Theregulation signal, sat_bo, indicates the amount of power back-offrequired and can be expressed as:

BO+k_(BO∂V) _(apc)(n)

Where: BO is a fixed constant back-off value,

-   -   k_(BC) is a scalar value, and    -   ∂V_(apc)(n) is the deviation of the sampled control voltage        signal.

k_(BO) is selected such that the product k_(BO)∂V_(apc)(n) is adynamically weighted back-off step size that is inversely proportionalto the control slope of the control loop 104, i.e. the “smaller” thecontrol slope, the larger the back-off step. k_(BO) is empiricallydetermined prior to use of the power amplifier 114 and stored in alook-up table for use by the secondary control unit 130. The criterionfor selecting a value for k_(BO) is that the product k_(BO)∂V_(apc)(n)should result in a back-off such that the control slope is set to itsminimum value selected. The use of the constant back-off value, BO, thusensures that the control slope is always adjusted to above the minimumvalue therefor. In this example, the back-off step set by the regulationsignal, sat_bo, is 0.5 dBm, constituting a first power back-off 162.

This first power back-off 162 succeeds in causing the deviation of thecontrol voltage signal, ∂V_(apc), to return to a level below thethreshold, V_(T), in respect of a fourth perturbation 156 and a fifthperturbation 158. However, due to the slope droop mentioned above, thesixth perturbation 160 again eventually exceeds the threshold value,V_(T), and so the regulation signal, sat_bo, generated by the secondarycontrol unit 130 causes the reference ramp generator 100 to back thecontrol voltage signal, V_(apc), off by a first quantum and hence thepower output of the power amplifier 114 too. In this example, backingthe control voltage signal, V_(apc), off by a second quantum results inthe second power back-off 164, i.e. in this example a further reductionof 0.5 dBm to the output power of the amplifier 114. This processcontinues in order to maintain the deviation of the control voltagesignal, ∂V_(apc), below the threshold value, V_(T), and hence preventcollapse of the control slope and the bandwidth of the control loop 104.However, in this example, a settling time is set to elapse betweensuccessive back-offs, because the control loop 104 is dynamic and doesnot respond instantaneously to a back-off. After each back-off a time istherefore allowed for the control loop 104 to respond to the back-off.To do otherwise would, in this example, result in repeated back-offsbeing triggered in quick succession causing an unacceptable drop in theoutput power of the power amplifier 114.

In extreme saturation conditions, where the control voltage signal,V_(apc), ramps up and remains at an upper limit thereof prior toapplication of the perturbations (as mentioned above), the deviation ofthe control voltage signal, ∂V_(apc), is not detectable and so thecontrol slope cannot be measured. In order to obviate this limitation, athreshold can be set in respect of the magnitude of the control voltagesignal, V_(apc), so that a power back-off is triggered when themagnitude of the control voltage signal, V_(apc), exceeds the threshold.In this example, the (upper) threshold is a design time-defined constantclose to an upper limit of an output of the Sinc filter 122 and/or theADC 120. If the control slope of the control loop 104 falls below thepre-set threshold level, V_(T), the power amplifier 114 is deemed tohave saturated and so a back-off is required. In this case, it isassumed that the control voltage signal, V_(apc), will never, during acurrent time slot, need to exceed a current value of the control voltagesignal, V_(apc). The current value of the control voltage signal,V_(apc), is therefore sampled and used as the upper threshold for thecontrol voltage signal, V_(apc), for the remainder of the time slot. Fora subsequent time slot, the upper threshold for the control voltagesignal, V_(apc), reverts back to the design time-defined value.

In order to avoid multiple back-offs for different reasons, theprovision of the perturbations is temporarily disabled, as alreadyexplained above, when an extreme saturation condition occurs.

Whilst in the above example, the control voltage signal, V_(apc), isbacked off, other parameters of the power amplifier can be modified inorder to maintain the control slope within the desired range, forexample adjusting one or more parameter of a tuning circuit associatedwith the power amplifier 114. Of course, in the general case, anysuitable parameter capable of maintaining the control slope within thedesired range can be modified in relation to the unit under control.

It is thus possible to provide a method and apparatus capable ofregulating a control loop in order to avoid collapse of a control slopeand a bandwidth associated with the control loop. Furthermore, thecontrol loop can be implemented as a single loop. Additionally, thedeviation of the control signal is inversely proportional to the controlslope and so as the control slope diminishes, the magnitude of thedeviation increases, thereby resulting in an improved signal-to-noiseratio in respect of the deviation of the control signal being measured.Of course, the above advantages are exemplary, and these or otheradvantages may be achieved by the invention. Further, the skilled personwill appreciate that not all advantages stated above are necessarilyachieved by embodiments described herein.

1. A method of regulating a control slope of a control loop associatedwith a unit under control, the method comprising the steps of: measuringthe control slope of the control loop; modifying a parameter associatedwith the unit under control in order to maintain the control slopewithin a desired range, thereby maintaining lock of the control loop;generating a reference signal having an information contentcorresponding to a profile that an output of the unit under control isrequired to follow; generating a control signal in order to control theoutput of the unit under control; measuring the control slope bymeasuring a frequency response between the reference signal and thecontrol signal.
 2. A method as claimed in claim 1, further comprisingthe steps of: providing the control signal with a perturbation.
 3. Amethod as claimed in claim 2, wherein the perturbation is distinct fromthe information content corresponding to the profile.
 4. A method asclaimed in claim 2, wherein the perturbation is an amplitude modulationperturbation.
 5. A method as claimed in claim 1, further comprising thesteps of: modifying the parameter associated with the unit under controlby a first quantum.
 6. A method as claimed in claim 5, furthercomprising the step of: modifying the parameter associated with the unitunder control by a second quantum in response to the control slope beingoutside the desired range, the parameter associated with the unit undercontrol being modified by the second quantum after elapse of a delayperiod.
 7. A method as claimed in claim 5, wherein the reference signalis generated by a scaling process, the method further comprising thesteps of: modifying the parameter associated with the unit under controlby modifying the scaling process.
 8. A method as claimed in claim 1,further comprising the steps of: measuring the control slope bymeasuring a rate of change with time of the control signal.
 9. A methodas claimed in claim 8, wherein the rate of change with time of thecontrol signal is measured by band-pass filtering the control signal.10. A method as claimed in claim 9, further comprising the steps of:band-pass filtering the control signal by low-pass filtering followed byhigh-pass filtering the control signal.
 11. A method as claimed in claim1, wherein the profile has an active portion, the control slope beingmeasured in respect of the active portion.
 12. A method as claimed inclaim 1, further comprising the steps of: independently evaluating amagnitude of the control signal relative to a threshold value in orderto determine saturation of the control signal.
 13. A method as claimedin claim 1, wherein the unit under control is a power amplifier.
 14. Amethod as claimed in claim 11, wherein the parameter associated with theunit under control is a bias control of the power amplifier.
 15. Amethod as claimed in claim 1, further comprising the steps of: providinga feedback path from the control loop in order to measure the controlslope of the control loop.
 16. A method as claimed in claim 15, whereinthe feedback path is disableable when the control slope is not beingmeasured.
 17. A method as claimed in claim 15, wherein the control loopis arranged to operate in an analogue domain and the feedback path isarranged to operate in a digital domain.
 18. A method as claimed inclaim 15, wherein the unit under control has a tuning circuit associatedtherewith, the method further comprising the step of: modifying theparameter associated with the unit under control by modifying aparameter of the tuning circuit.
 19. A control slope regulationapparatus for a control loop associated with a unit under control, theapparatus comprising: a processing circuit arranged to measure, when inuse, a control slope for the control loop and generate a regulationsignal for modifying a parameter associated with the unit under control,the regulation signal being used to maintain the control slope within adesired range for maintaining lock of the control loop; wherein; theprocessing circuit is further arranged to generate a reference signalhaving an information content corresponding to a profile that an outputof the unit under control is required to follow; and the control loopgenerates a control signal in order to control the output of the unitunder control; the control slope is measured by measuring a frequencyresponse between the reference signal and the control signal.
 20. Amethod as claimed in claim 2, further comprising the steps of: modifyingthe parameter associated with the unit under control by a first quantum.